British Pat. No. 973,377 discloses a family of ester imide resins made by reacting together at least one polybasic acid or a functional derivative thereof, and at least one polyhydric alcohol or functional derivative thereof, at least one of the reactants having at least one five-membered imide ring between the functional groups of the molecule. It further discloses that the reactants can be heated in a commercial cresol mixture, then further diluted in a mixture of naphtha and cresol and used as an enamel for coating copper wire to produce a hard, thermally resistant insulation therefor. U.S. Pat. No. 3,426,098 describes ester imide resins in which all or part of the polyhydric alcohol comprises tris(2-hydroxyethyl) isocyanurate. U.S. Pat. No. 3,382,203 describes polyester wire coating enamels based on mixed esters containing aromatic polycarboxylic acid radicals and radicals of di- and/or polyhydric alcohols, and specifically contemplates the inclusion of imide containing units. A preference is stated for cresol as a solvent. U.S. Pat. No. 3,274,159 describes polyester polyimides of trimellitic anhydride in which a glycol reactant is stated to be present in excess of stoichiometric proportions when no solvent is employed in the polymerization reaction. Such polyester polyimides are stated to be useful as outdoor wire insulation material. U.S. Pat. No. 3,141,859 acknowledges that certain esters and ethers of ethylene glycol and polyethylene glycol can be used in the preparation of alkyd resins to make wire enamels, but it is not disclosed if such compounds are mono- or di- ethers and esters, and, in any event, the statement is made that "completely satisfactory wire enamels have not been obtained by such a process." The disclosures of the foregoing patents are incorporated herein by reference.
Keating, U.S. Pat. Nos. 4,119,605, 4,119,608 and 4,119,758, incorporated herein by reference, discloses polyester-imide resins which are soluble in non-phenolic solvents by incorporating a monoether of a diethylene glycol or a triethylene glycol into the resin. It is stated therein that the use of the monoether of a glycol as a reactant which serves as a monofunctional alcohol serves to control the molecular weight and solubility of the polymer and permits the manufacture of wire enamels which contain relatively non-toxic solvents in place of phenols and also permits the attainment of enamels having economical high solids contents. The reactive glycol dealt with by the above-mentioned Keating patents is an ether alcohol which either (a) has the formula RO(C.sub.n H.sub.2n O).sub.x H where R is an alkyl group of 1 to 6 carbon atoms or phenyl, n is 2 or 3 and x is 2 or 3 or (b) is the monophenyl ether or ethylene glycol. It is seen that mixed glycols, i.e., those wherein x is 2 and 3 are specifically not contemplated by Keating.
Enamels prepared with such reactive monohydric alcohols are not entirely satisfactory in comparison with conventional polyester imides because, while reacting the ether alcohol into the resin lowers its molecular weight and improves solubility, it also tends to reduce ultimate physical properties precisely because the molecular weight has been lowered.
It has now been discovered that if polyester imide resins are prepared in the presence of a monobutyl ether of a partially linear, partially branched aliphatic diglycol having 2 carbon atoms in one portion and 3 carbon atoms in the other portion of the aliphatic chain and in which the monohydric alcohol group is a secondary alcohol group, e.g., 1-butoxyethoxy-2-propanol, then there is little tendency for the glycol ether to react into the resin, and the molecular weight is higher than it would be if a glycol ether of the type described in the Keating patents is employed. The result is that a superior wire enamel can be made by dissolving such higher molecular weight resins in a solvent comprising predominantly the same, or different, polar organic monoethers or monoesters of alkylene glycols or polyalkylene glycols. The present enamels are clear solutions, even without chemically binding in the glycol ether. They thus eliminate the need for cresylic acid solvent and therefore provide excellent results without any toxic effluents.